The invention relates to a probe for a capacitive sensor device with an outer sleeve and at least one probe head arranged within the outer sleeve, the probe head comprising a measuring element with at least one measuring and front face, the measuring element being made of a metal, a metal alloy or an electrically conductive ceramic or being coated at least at the measuring and front face with a metal, a metal alloy or an electrically conductive ceramic and a first electrically non-conductive isolator element and a first partial element of a first shield, wherein the first partial element is made of a metal, a metal alloy or an electrically conductive ceramic. The invention also relates to a gap measuring system for determining a rotor gap.
These types of probes are already known from the prior art and are used in a wide variety of industrial measuring applications. These types of probes measure distances of movable parts relative to one another, such as components of machines, particularly turbomachines of all types. In particular, in the case of turbomachines, the size of the distance or the gap between a rotor blade and the surrounding housing has an effect on so-called leakage losses, which in turn impact the efficiency of the machine. Capacitive probes are known from German Patent Document No. DE 34 33 351 C1 and European Patent Document No. EP 0 246 576 B1. These probes feature a triaxial structure and are used in particular for measuring the gap between a housing and corresponding rotor blades in aircraft gas turbines, wherein the runtime signal of the probes is used for non-contacting blade oscillation measurement. Due to the triaxial structure of these known probes and the thereby induced enclosure of different materials, such as for example, ceramic and metal or metal alloys with different coefficients of thermal expansion, cracks may occur when using these types of probes at higher temperatures. Particularly when using these types of probes in gas turbines of aircraft engines, service lives of 10,000 hours and more are required, during which time the probes must supply reliable measured values of the rotor gap. In this case, depending on the concrete design of the turbomachine, the probe is exposed to varying temperatures of up to greater than 700° C., high pressures, oscillations and other stresses from water, salt, oil, dirt, metallic abrasion and the like. To avoid temperature-induced stress cracks, German Patent Document No. DE 60 2004 004 909 T2 discloses a sensor for the capacitive measurement of the distance to a stationary object or one passing by, in which all elements are configured of electrically conductive or electrically non-conductive ceramic materials and the materials are selected in such a way that they have similar coefficients of thermal expansion. However, the disadvantage of this sensor is that material selection is very restricted on the one hand, which results in the design and manufacture of this sensor being complex and expensive. In addition, because of the restricted material selection, the measuring precision of the sensor is not guaranteed over the entire service life of the sensor.
As a result, the object of the present invention is making available a probe and a gap measuring system of the type cited at the outset, which is useable at high temperatures and guarantees measuring precision as well as a long service life.
Advantageous embodiments of the probe, to the extent to which they may be applicable, should be viewed as advantageous embodiments of the gap measuring system and vice versa.
A probe for a capacitive sensor device according to the invention has an outer sleeve and at least one probe head arranged within the outer sleeve, wherein the probe head comprises a measuring element with at least one measuring and front face, the measuring element being made of a metal, a metal alloy or an electrically conductive ceramic or being coated at least at the measuring and front face with a metal, a metal alloy or an electrically conductive ceramic and a first electrically non-conductive isolator element and a first partial element of a first shield, wherein the first partial element is made of a metal, a metal alloy or an electrically conductive ceramic. In this case, the measuring element, the first isolator element and the first partial element of the first shield are adhesively connected to one another and configured as a multilayer, the first isolator element being disposed between the measuring element with the measuring and front face and the first partial element. Because of the inventive layered structure of the probe head and the adhesive connection of the individual elements of the probe head among one other, thermal expansion of the individual elements is possible in an almost unhindered manner. In particular, a sheathing of the electrically non-conductive materials by the electrically conductive materials and vice versa is avoided so that even with different coefficients of thermal expansion of the materials, it is not possible for stress cracks to occur. Because non-positive or positive connections are not used, the probe is also mechanically stable over a wide temperature range. Because it is possible to use metal or metal alloys to form the measuring element or the measuring and front face of the measuring element, this results in a high level of measuring precision with respect to possible changes in capacitance. A very long service life for the probe is guaranteed by the high level of mechanical stability and the possibility of thermal expansion of the different materials. In a preferred embodiment of the invention, the layering of the probe head that is configured as a multilayer is configured to be approximately perpendicular to a longitudinal axis of the probe.
In other advantageous embodiments of the probe according to the invention, the probe head is arranged in a shielding sleeve made of metal, a metal alloy or an electrically conductive ceramic and surrounded by the shielding sleeve, wherein the shielding sleeve is adhesively connected to the first partial element. Because of the shielding sleeve, the non-homogenous boundary area of the electrical field of the probe head, in particular the measuring and front face of the measuring element, is shielded. This results in an approximately parallel electrical field between the probe and a counter electrode opposite from the measuring and front face, whose changes in capacitance can be detected with a high level of measuring precision. In addition, peripherally occurring interference areas can be shielded reliably. Furthermore, the probe may have a probe body arranged within the outer sleeve, wherein the probe body has a second partial element with the first shield made of a metal, a metal alloy or an electrically conductive ceramic, a second electrically non-conductive isolator element and a first partial element of the second shield, wherein the first partial element is made of a metal, a metal alloy or an electrically conductive ceramic. In this case, the second partial element of the first shield, the second isolator element and the first partial element of the second shield are also adhesively connected to one another and configured as a multilayer, wherein the second isolator element is disposed between the second partial element of the first shield and the first partial element of the second shield. Because of this type of embodiment of the probe body, a high level of mechanical stability of the probe body or of the probe is again guaranteed as a whole over a wide temperature range. The layered structure of the probe body in turn makes possible the almost unhindered thermal expansion of the individual elements of the probe body, thereby avoiding temperature-induced cracks. In particular, again a sheathing of the electrically non-conductive materials by the electrically conductive materials and vice versa is avoided so that even with different coefficients of thermal expansion of the materials, it is not possible for stress cracks to occur.
In another advantageous embodiment of the probe according to the invention, the first partial element and the second partial element of the first shield are adhesively connected to one another. This produces a high level of mechanical stability for this connection between the probe head and the probe body. In addition, it is possible also for the layering of the probe body that is configured as a multilayer to be configured approximately perpendicular to the longitudinal axis of the probe.
In another advantageous embodiment of the probe according to the invention, the first partial element of the second shield is at least partially adhesively connected to a second partial element of the second shield, wherein the second partial element is also made of metal, a metal alloy or an electrically conductive ceramic and configured as a holder for the probe body and the therewith connected probe head within the outer sleeve. The adhesive connection of the partial elements of the second shield in turn guarantees a high level of mechanical stability of this element over a wide temperature range. In addition, the second partial element may advantageously have at least one shoulder, which is located at least partially circumferentially on its outer circumference, for bearing on at least one projection configured on the inner circumference of the outer sleeve. This guarantees a secure connection and positioning of the probe within the outer sleeve.
In another advantageous embodiment of the probe according to the invention, a radially circumferential gap is configured between the inner circumference of the outer sleeve and the shielding sleeve. In addition, it is possible for the gap to extend at least into the region of the second isolator element. The radially circumferential gap advantageously supports the expansion possibilities of the different materials of the probe, in particular the probe head, the probe body and the shielding sleeve in the case of a corresponding temperature effect. A thermal expansion is readily possible, temperature-induced stress cracks are reliably prevented.
In other advantageous embodiments of the probe according to the invention, the first and the second isolator element are made of a non-conductive ceramic, in particular AL2O3 or glass. In addition, the materials for the measuring element with the measuring and front face, the elements of the first and second shield and the first and second isolator element may be selected such that they have similar coefficients of thermal expansion. This measure also prevents possible temperature-induced stress cracks within the probe. In this case, particularly the material for the measuring element with the measuring and front face and the elements of the first and second shield may be an iron-nickel-cobalt alloy with a low coefficient of thermal expansion. These types of materials are known for example, under the trade names of Vacon 11 and ALLOY42. In addition, the elements of the probe head and/or the elements of the probe body may be vacuum soldered, wherein silver titanium is preferably used as the soldering material. All other connections between elements of the probe may be configured to be laser welded also as an adhesive connection.
In another advantageous embodiment of the probe according to the invention, the probe is connected to a triaxial cable, wherein an inner conductor of the triaxial cable is connected in an electrically conductive manner to the measuring element or the measuring and front face, a center shield of the triaxial cable is connected in an electrically conductive manner to the second partial element of the first shield and an outer shield of the triaxial cable is connected in an electrically conductive manner to the second partial element of the second shield. In this case, passage openings are configured along the longitudinal axis of the probe for the passage of the inner conductor, of the center shield and of the outer shield of the triaxial cable. In an advantageous embodiment, the diameter of the passage opening in the region of the first isolator element and/or the first partial element of the first shield is larger at least in sections than the diameter of the inner conductor. In addition, it is possible for the diameter of the passage opening in the region of the second isolator element and/or the first partial element of the second shield to be larger at least in sections than the diameter of the center shield. The passage enlargements guarantee that the material surrounding the passage openings or the surrounding materials may expand relatively unhindered in the case of the effect of temperature. As a result, a possible crack formation due to different coefficients of thermal expansion of the materials of the individual elements is reliably counteracted also in these regions of the probe.
In another advantageous embodiment of the probe according to the invention, the passage opening arranged on the end of the second partial element of the second shield that faces away from the measuring and front face is configured to be asymmetrically funnel-shaped with respect to the lateral cable exit and cable entry of the triaxial cable. The lateral cable exit and cable entry of the triaxial cable advantageously reduce the construction height of the probe according to the invention.
In another advantageous embodiment of the invention, the probe head is configured conically, wherein it tapers in the direction of the measuring and front face, and the shielding sleeve has at least one projection formed on its inner circumference to hold the probe head. As a result, this guarantees a secure positioning and fastening of the probe head within the shielding sleeve.
In other advantageous embodiments of the probe according to the invention, the probe or the probe body and/or the probe head is disposed in a spacer sleeve. As a result, the distance from the object being measured can be configured in a variable manner, and additionally, the probe may be readily calibrated because of modified distances. In particular, the outer sleeve of the probe may be configured as a spacer sleeve, wherein the sleeve may then be clamped to the surrounding housing of the corresponding component. The probe may also be welded to the housing.
A gap measuring system according to the invention for determining a rotor gap between a rotor comprising rotor blades and a rotor housing, which surrounds at least sections of the rotor or rotor blades, of a turbomachine features at least one capacitive sensor device having an electrode and a counter electrode for determining the capacitance measured values that characterize the rotor gap, wherein the rotor or the rotor blades is/are switchable as the counter electrode of the sensor device and the electrode is a probe according to the invention, as has been described in the foregoing. In this case, the turbomachine may be in particular a gas turbine of an aircraft engine or even a stationary gas turbine, a steam turbine or a turbocharger. Using a probe according to the invention for the gap measuring system guarantees that the system is useable at high temperatures and has a high level of measuring precision as well as a long service life.
Additional advantages, features and details of the invention are disclosed on the basis of the following description of two exemplary embodiments as well as on the basis of the drawings.